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Are the wild horses of the American west native?

THE wild horses of the American west should be considered a native species as fully deserving of protection as elk or antelope, even though they are the descendants of domestic livestock introduced by European settlers. Or so claim animal rights groups.

In a case before a US federal court they are arguing that wild horses should no longer be rounded up to make room for cattle. If the claim is successful, it could change the way the US Bureau of Land Management (BLM) manages tens of thousands of wild horses on federal lands.

“Right now, the BLM treats them as if they are a nuisance or livestock. They deserve more respect than that,” says Rachel Fazio, the lawyer representing In Defense of Animals and other plaintiffs in the suit.

In the plaintiffs’ favour, there is no doubt that North America is the ancestral home of horses. The horse family evolved there, and it is almost certain that the modern species, Equus caballus, was present in North America for almost 1.5 million years before dying out about 10,000 years ago.

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On the other hand, today’s wild horses are the feral descendants of domestic horses from Europe, with a 6000-year history of domestication. Whether these horses can be considered truly native thus hinges on whether a few millennia of foreign domestication are enough to “spoil” 1.4 million years of native evolution. Animal rights groups are adamant that they are not, but the BLM’s website labels as “false” any claim that the horses can be considered native.

Settling the dispute will be difficult, especially because no wild, never-domesticated E. caballus survive for comparison. Pleistocene horse fossils exist, though, and the tiny amount of DNA that has been recovered from them shows no clear differences from today’s wild horses, says Jacobo Weinstock at the University of Southampton, UK.

Still, both fossils and ancient cave paintings suggest that modern horses look rather different from their Pleistocene ancestors – taller, more slender and longer-legged. And, like all domesticated animals, their brains are smaller, says Melinda Zeder at the Smithsonian Institution in Washington DC. However, the mantle of domestication seems to sit more lightly on horses than other domesticated animals. Zeder says their brains have shrunk less – only about 14 per cent – than animals such as pigs, which have brains around 34 per cent smaller than boars. Moreover, feral horses develop highly structured social systems and do well in the wild – unlike feral dogs, which hang around the periphery of human society and depend on its refuse.

Yet according to some researchers, whether or not the horses are native ought to be irrelevant to their treatment by the BLM. Mark Davis at Macalester College in Saint Paul, Minnesota, is lead author of a new paper that questions the valuing of native species over non-native ones (Nature, DOI&colon; 10.1038/474153a). He says the distinction between native and introduced is arbitrary. “The question should be, are wild horses causing a problem? Are they providing benefits? Then you can develop policy to either reduce or increase their numbers.”